Urethane-containing oxazolidone-modified isocyanurate foams containing a relatively large amount of polyurethane are known in the art. By way of illustration, U.S. Pat. No. 3,849,349, issued Nov. 19, 1974 to K. C. Frisch et al discloses such foams made by a one-step reaction. In this reaction, a polynuclear polyisocyanate having an isocyanate equivalent weight of between 120 and 160, a polyepoxide, and a polyol are reacted in the presence of a blowing agent and an isocyanate trimerization catalyst. The polyol is present in the foam formulation in an amount of between 15 and 40 percent based on the total weight of these three reactants. The foam made in accordance with this patent contains a significant amount of thermally unstable urethane linkages produced by the reaction of the polyol and the polyisocyanate, thereby making this foam subject to a relatively large amount of thermal degradation at elevated temperatures.
Urethane-free oxazolidone-modified isocyanurate foams characterized by high temperature degradation resistance are disclosed in a co-pending application, U.S. Ser. No. 855,992, of S. Fuzesi et al assigned to Olin Corporation. These urethane-free foams, although exhibiting excellent thermal degradation resistance, sometimes exhibit cracking during high temperature processing, particularly mold-fabricated foams.
Hayash et al, U.S. Pat. No. 3,673,138 discloses polyoxazolidone foams produced by reacting a polyepoxide with a polyisocyanate in a proportion of from about 0.70 equivalent to about 1.25 equivalents of polyepoxide per equivalent of polyisocyanate. However, the use of the such high relative amounts of polyepoxide is costly since commercial polyepoxides are about twice as expensive as polyisocyanates. Moreover, the use of these ratios of polyepoxide to polyisocyanate have been found to produce such a strong exothermic reaction as to cause scorching and associated thermal degradation of interior portions of the foam during fabrication thereof.
In view of the above, it would be highly desirable to provide improved foams characterized by excellent thermal degradation resistance, as well as scorch and crack resistance during high temperature aging of the foams.